There is already a known process for the preparation of isobutyryl fluoride from an anhydrous mixture of propylene, carbon monoxide, and hydrogen fluoride In particular, U.S. Pat. No. 4,499,029 discloses passing such a mixture through at least two reaction zones arranged in series and adding to the reaction mixture, between the reaction zones, progressive quantities of anhydrous propylene and carbon monoxide, the process being carried out with a residence time of 15 seconds to 10 minutes in the reaction zones, at a pressure of 1 to 150 bars and a temperature of 0.degree. to 100.degree. C., the molar relationship C.sub.3 H.sub.6 /CO/HF in the reaction mixture being between 1/5/5 and 1/30/200.
However, the cost of manufacture of isobutyryl fluoride produced in this manner is fairly high, given that the starting material in this process, namely propylene, is itself produced by dehydrogenation of alkanes from oil cuts or by steam cracking of hydrocarbons.
It is already known, from H. Hogeveen and C. F. Roobeek, Rec. Trav. Chim. Pays-Bas, 91 (1972), pages 137-40, to react at 0.degree. C. an equimolar mixture of n-butane and carbon monoxide in the presence of antimony pentafluoride SbF.sub.5 in solution in SO.sub.2 ClF. This reaction leads to the formation of a mixture of sec-butyloxocarbonium (74%), tert-butylcarbonium (25%) and tert-butyloxocarbonium (1%) ions. From the same document, it is known to carbonylate propane at 0.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of antimony pentafluoride, the molar relationship C.sub.3 H.sub.8 /CO being between 1 and 9. It is also known, from N. Yoneda et al, Chemical Letters (Chemical Society of Japan), (1983), pages 17-18, to carbonylate branched alkanes containing at least 5 carbon atoms, at a temperature of 30.degree. C., in the presence of the superacid HF-SbF.sub.5 (molar relationship HF/SbF.sub.5 equal to 5), the molar relationship alkane/HF being equal to 0.1. Furthermore, it is known from G. Olah et al, Journal of the American Chemical Society, 95, pages 4939 et seq., that:
at a temperature between -10.degree. C. and -103.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of the superacid HSO.sub.3 F--SbF.sub.5, an equilibrium is established between propane and the isopropyl cation, and PA1 at a temperature of -78.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of a superacidic system comprising hydrogen fluoride and antimony pentafluoride, an equilibrium is established between 2-methylpropane (or isobutane) and the trimethylcarbenium ion. PA1 As used herein: PA1 "principally alkane" means that the aliphatic hydrocarbon stream may contain, besides the alkane (propane, n-butane, or isobutane), small proportions of alkenes or alkynes containing a low number of carbon atoms, such as particularly butene, propyne, or propylene; it should further be understood that isobutyryl fluoride is obtained from propane, methyl-2 butyryl fluoride from n-butane, and pivaloyl fluoride from isobutane. PA1 "principally alkyloxocarbonium cation" means that the reaction generated by the superacidic catalyst system according to the invention leads essentially to the formation of this cation, in addition to minor proportions of carbocations that either are derived from the other aliphatic hydrocarbons that may be present in the reaction mixture or that originate from rearrangements of the carbocations or of the alkyloxocarbonium cation. Thus, the isopropyloxocarbonium (also called isobutyryl) cation is formed from propane, the sec-butyloxocarbonium (also called 2-methylbutyryl) cation is formed from n-butane, and the tertbutyloxocarbonium (also called pivaloyl) cation is formed from isobutane; and PA1 "iodide" or "bromide" means an ionic compound in which at least one iodine or bromine atom is linked to a metal atom or else to an organic group; examples of such compounds are particularly alkali metal, alkalineearth metal and quaternary ammonium iodides and bromides. PA1 in the case of the 2-methylbutyryl cation: a triplet at about 1.25 ppm (3H), a doublet at about 1.85 ppm (3H), a multiplet at about 2.3 ppm (2H) and a sextuplet at about 4.15 ppm (1H); PA1 in the case of the isobutyryl cation: a doublet at about 2.1 ppm (6H) and a heptuplet at about 4.4 ppm (1H); and PA1 in the case of the pivaloyl cation: a singlet at about 2.0 ppm (9). PA1 a molar relationship CO/alkane at least equal to 1.5 and preferably of between approximately 2 and 30, PA1 a molar relationship HF/SbF.sub.5 of between approximately 1 and 30, PA1 a temperature between approximately -80.degree. C. and +60.degree. C., PA1 a proportion of bromine or bromide, relative to SbF.sub.5, of between approximately 0.1 and 5 mole %, and PA1 a proportion of iodine or iodide, relative to SbF.sub.5, of between approximately 1 and 10 mole %.
E. Hogeveen has already described in Adv. Phys. Org. Chem., 10, 32 (1973) the decarbonylation reaction of the pivaloyl cation at -70.degree. C., either in an equimolar mixture of hydrogen fluoride and antimony pentafluoride or in a mixture of 2 parts by volume of SO.sub.2 ClF per 1 part by volume of antimony pentafluoride, to form the tert-butyl cation.
It will be noted that, in general, these prior documents are concerned exclusively with the kinetics of protonation of alkanes or of decarbonylation at a very low temperature and do not describe any covalent species capable of being obtained by employing these reactions In particular, none of them describe the production of acid fluorides. Furthermore, none of them have demonstrated the possibility of regenerating the superacid employed for the protonation
Furthermore, U.S. Pat. No 4,582,571 mentions the possibility of forming isobutyryl fluoride by reaction of carbon monoxide, propane, anhydrous hydrogen fluoride, and antimony pentafluoride at a pressure above 100 bars and at a temperature close to 100.degree. C. However, on the one hand this reference is silent with respect to the proportions of the various reactants, and on the other hand experiments have shown that, under these reaction conditions, a small proportion of isobutyryl fluoride is actually formed alongside a major proportion of propionyl fluoride. A process of this kind is therefore not capable of being employed industrially when it is desired to form chiefly isobutyryl fluoride, from propane